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Homework Help: Newtons third law pulleys

  1. Oct 17, 2007 #1
    Please help me because my teacher has not explained how to do questions involving angles.

    Blocks X and Y of masses mx=5.12kg and my=3.22kg are connected by a fishing line passing over a frictionless pulley. Show that block X slides up the incline (35.7 degrees above the horizontal) with positive acceleration. Determine the magnitude of the acceleration. (0.273m/s2 is the answer)

    i want to find the gravitational force for mass x so Fg=5.12x9.8=50.2N
    For mass y f=mg =3.22x9.8=31.6N

    Just like with projectile problems you need to find the x and y components of Ftension for block x,which is getting pulled up at an angle but how do i do that if i dont know the applied force?
    Last edited: Oct 17, 2007
  2. jcsd
  3. Oct 17, 2007 #2


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    So the block y is hanging vertically?

    Write the [tex]\Sigma F=ma[/tex] equation for the block y, and the [tex]\Sigma F=ma[/tex] equations for block x

    Call the tension T. The acceleration of block y downward equals the acceleration of block x up the plane. Call this a.

    Is the incline frictionless?

    You have 2 unknowns T and a which you should be able to solve for with the equations you get.
  4. Oct 17, 2007 #3
    yes, the incline is frictionless and block y is hanging vertically. if i know that the normal force of y is 31.6N, does that help me in any way?
  5. Oct 17, 2007 #4


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    If block y is hanging... how is there a normal force?

    The way you described the problem... there are 2 forces acting on y... the weight, and tension... write the [tex]\Sigma f = ma[/tex] equation for y.
  6. Oct 17, 2007 #5
    oops... i guess i meant the gravitational force is 31.6N.
  7. Oct 17, 2007 #6
    if i only have the weight of block y how am i able to find out FT? After all, isnt FT calculated by saying


    do i not have 2 variables here?
  8. Oct 17, 2007 #7


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    yes... I'll call FT, T...

    Fnet = T - my*g

    now... I'm going to take a as the downward acceleration... taking up as positive and down as negative:

    my*(-a) = T - my*g (which has two variables, a and T)

    can you come up with an equation for block x?
    Last edited: Oct 17, 2007
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